Enable topological editing on first TopoGeometry layer creation

I'm not sure this is an acceptable behavior, but I'm sure users
do want to edit TopoGeometry layers in a topological way.
The problem with this implementation is that topological editing
is enabled for editing of every layer, not just the TopoGeometry
one.

src/providers/postgres/qgspostgresprovider.cpp

@@ -23,6 +23,7 @@
 #include <qgsmessagelog.h>
 #include <qgsrectangle.h>
 #include <qgscoordinatereferencesystem.h>
+#include <qgsproject.h>
 
 #include "qgsvectorlayerimport.h"
 #include "qgsprovidercountcalcevent.h"
@@ -132,6 +133,9 @@ QgsPostgresProvider::QgsPostgresProvider( QString const & uri )
       disconnectDb();
       return;
     }
+
+    // Enable topological editing
+    QgsProject::instance()->writeEntry( "Digitizing", "/TopologicalEditing", true );
   }
 
   mLayerExtent.setMinimal();